The present application claims priority to Japanese Application No. P2000-232821 filed Aug. 1, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a process for crystal growth used to form semiconductor light-emitting devices. More particularly, the present invention relates to a process for selectively growing crystals of nitride semiconductor.
Nitride semiconductors such as gallium nitride compound semiconductors are formed by means of vapor phase growth. Vapor phase growth needs a special process so that it yields high-quality crystals with a low dislocation density, as disclosed in Japanese Patent Laid-open No. Hei 10-312971. According to this disclosure, the process consists of depositing gallium nitride to form a first semiconductor layer, forming a protective film from a material (such as silicon oxide and silicon nitride) which prevents the growth of gallium nitride compound semiconductor, and allowing the gallium nitride compound semiconductor to grow in the lateral direction or in-plane direction from the region which is not covered by the protective film, thereby forming a second semiconductor layer. This layer structure prevents the propagation of the threading dislocations which extend vertically from the substrate interface.
A similar technique is disclosed in MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999). According to this disclosure, the reduction of threading dislocations is accomplished by the steps of growing a first gallium nitride compound semiconductor layer, selectively removing the semiconductor layer by reactive ion etching (RIE for short hereinafter), and selectively growing a second gallium nitride compound semiconductor layer from the remaining crystals. These techniques yield crystal film having a low dislocation density (about 106 cmxe2x88x922), and they have realized long-life semiconductor lasers.
The former technique mentioned above consists of steps of forming a first gallium nitride compound semiconductor layer, forming a protective film, and forming a second gallium nitride compound semiconductor layer. This technique minimizes the propagation of threading dislocations into the second gallium nitride compound semiconductor layer from the underlying layer if the protective film is formed such that the area of windows or the area of uncovered regions in the protective film is as small as possible. In practice, however, the selective growing needs a comparatively high growing temperature (1000xc2x0 C. or higher) so that deposition on the protective film is as small as possible and because the ordinary growing temperature of GaN is 1000xc2x0 C. or higher. This high temperature causes vigorous evaporation, retarding the start of crystal growth or growing crystals unevenly if the area of the window region is small. This presents difficulties in achieving uniformity and good reproducibility. Therefore, it is impracticable to reduce the area of the window region and there is a limit to the reduction of the threading dislocation density in the second gallium nitride compound semiconductor layer.
It is an object of the present invention to provide a process for vapor phase growth of nitride semiconductor involving a protective film. This process grows high-quality semiconductor crystals uniformly and invariably even though the protective film is so formed as to give small uncovered window regions.
According to the present invention, the process for vapor phase growth of nitride semiconductor includes forming a first nitride semiconductor layer on a substrate, forming thereon a protective film for crystal growth prevention in such a way that it has partly open window regions through which the first nitride semiconductor layer is exposed, forming a second nitride semiconductor layer by selective growth from the first nitride semiconductor layer at a crystal growth starting temperature, and continuing crystal growth at a temperature higher than the crystal growth starting temperature.
According to the present invention, the process for vapor phase growth of nitride semiconductor comprises forming a first nitride semiconductor layer on a substrate, forming thereon a protective film for crystal growth prevention in such a way that it has partly open window regions through which the first nitride semiconductor layer is exposed, forming a second nitride semiconductor layer by selective growth from the first nitride semiconductor layer in such a state that the nitride semiconductor is released at a low level on the surface of the first nitride semiconductor layer or the second nitride semiconductor layer, and continuing crystal growth in such a state that the nitride semiconductor is released on the surface of the second nitride semiconductor layer at a higher level than at the time of start of crystal growth.
The present invention was developed in view of the fact that crystal growth which is usually carried out at a temperature at which crystals do not grow on the protective film or at a temperature at which the reactant gas evaporates does not proceed efficiently in the open window regions of the protective film. The process of the present invention is characterized in that the second nitride semiconductor layer is formed in two stages. The crystal growth is started at an adequate crystal growth starting temperature and is continued at a temperature higher than the crystal growth starting temperature; alternatively, the crystal growth is started in such a state that the nitride semiconductor is released at a low level on the surface of the nitride semiconductor layer and is continued in such a state that the nitride semiconductor is released on the surface of the nitride semiconductor layer at a higher level than at the time of start of crystal growth. In this way it is possible to suppress the evaporation of nitride semiconductor in the window regions.